Ocean thermal energy conversion (“OTEC”) is a method for generating electricity based on the temperature difference that exists between deep and shallow waters of a large body of water, such as an ocean. An OTEC system utilizes a heat engine (i.e., a thermodynamic device or system that generates electricity based on a temperature differential) that is thermally coupled between relatively warmer shallow and relatively colder deep water.
One heat engine suitable for OTEC is based on the Rankine cycle, which uses a low-pressure turbine. A closed-loop conduit containing a “working fluid” characterized by a low boiling point (e.g., ammonia) is thermally coupled with warm seawater at a first heat exchanger where the working fluid is vaporized. The expanding vapor is forced through the turbine, which drives a turbo-generator. After exiting the turbine, the vaporized working fluid is condensed back into a liquid state at a second heat exchanger where the closed-loop conduit is thermally coupled with cold seawater. The condensed working fluid is then recycled through the system.
OTEC systems have been shown to be technically viable, but the high capital cost of these systems has thwarted commercialization. The heat exchangers are the second largest contributor to OTEC plant capital cost (the largest is the cost of the offshore moored vessel or platform). The optimization of the enormous heat exchangers that are required for an OTEC plant is therefore of great importance and can have a major impact on the economic viability of OTEC technology.
One of the most efficient and cost-effective types of industrial heat exchangers is a plate-fin heat exchanger. Plate-fin heat exchangers can have higher surface area (due to their potential for high fin packing density) as compared to other types of heat exchangers, such as conventional tube and shell, plate-frame, etc. As a result, a plate-fin heat exchanger can have higher heat transfer efficiency, which makes it an attractive candidate for use in applications that require high fluid flow rates but are characterized by low temperature differentials, such as OTEC.
One of the highest efficiency plate-fin heat exchangers is the brazed-aluminum plate-fin heat exchanger, which comprises multiple layers of aluminum fins and plates that are made of materials having good thermal conductivity. The fins and plates are stacked and joined, using brazing, to form alternating passages for conveying fluids. In operation, fluids of different temperatures are passed through the alternating passages and heat energy is transferred between the fluids through the fin and plate materials.
Brazing is a well-known, low-cost process for joining mechanical elements. It is similar to soldering; however, brazing uses brazing-filler material that has a higher melting temperature (typically ≥450° C.) than traditional solder (˜250-300° C.). In many applications, brazing is preferred over soldering because brazing fillers have higher structural strength. In fact, brazed connections are often nearly as strong as the parts they connect, even at elevated temperatures.
In addition, complete assemblies comprising many brazed joints can be brazed at one time by arranging the assemblies, with brazing-filler material in place at each desired brazed joint. The entire arrangement is then heated at the same time, which induces the brazing-filler material to melt and fuse to its adjacent elements. As a result, the use of brazing offers significant cost advantages over many other joining technologies, such as fusion welding, etc.
Unfortunately, brazed joints are highly susceptible to galvanic-corrosion when exposed to a highly electrically conductive medium, such as seawater, geothermal fluid, mineral water, polluted water, and salt spray. The typical brazing process utilizes a filler aluminum alloy that has a lower melting temperature than the parent (base) metal being joined. Thus, the filler metal has different chemical composition and electric potential than the parent metal. The dissimilar metals, therefore, create a galvanic cell at the joint. Galvanic action at the joint induces metal migration (i.e., corrosion). In the presence of a conductive medium (e.g., seawater), the galvanic action at the joint is enhanced, which accelerates degradation of the joint. Furthermore, brazed joints that have failed because of galvanic-corrosion-related degradation usually cannot be reliably or cost-effectively refurbished.